Wellbores which are drilled in the earth formation for hydrocarbon exploration and production purposes become ever deeper and more complex in geometry as many times curved, inclined or horizontal sections are included. Such deep and complex wellbores impose stringent requirements on the drill strings used. However an unresolved problem is the occurrence of large friction forces between the drill string and the wellbore wall, which friction forces often hamper adequate wellbore operations.
For example it frequently occurs that the lower drill string part, generally referred to as the bottom hole assembly (BHA), becomes stuck in the wellbore. To release the stuck part of the drill string, a tensile- or compressive force is applied to the upper drill string part to free the stuck lower drill string part. To increase the effect of such force, a jarring tool is generally incorporated in the drill string at a location above the part of the drill string which is suspected to become stuck in the wellbore. Such jarring tool includes, for example, telescoping upper and lower parts, the upper part being connected to the upper drill string part and the lower part being connected to the lower drill string part. Upon applying a tensile- or compressive force to the upper drill string part, the upper telescoping part is initially subjected to a high resistance against upward or downward movement (for example by means of a narrow flow restriction for hydraulic oil) and thereafter suddenly to a low resistance against such movement until a stop prevents further movement. As a result, elastic energy which has initially accumulated in the upper drill string part is suddenly released and causes an impact force on the lower drill string part.
A problem of the conventional method of releasing a drill string is the occurrence of large longitudinal friction forces between the drill string and the wellbore wall, which friction forces significantly reduce the effective tensile- or compressive force at the stuck part of the drill string. Especially in highly deviated wellbores a large part of the tensile- or compressive force is counter-acted by longitudinal friction forces. Furthermore, the friction forces increase with increasing length of the drill string, so that for deeper wellbores it will be more difficult to free the drill string.
Furthermore, in wellbore drilling it is frequently required to clean the wellbore by removing drill cuttings from the wellbore using a stream of drilling fluid. However, a significant part of the drill cuttings sometimes cannot be efficiently removed.
It is an object of the present invention to provide a drill string tool and a method for significantly reducing the longitudinal friction forces imposed by the wellbore on the drill string.
It is another object of the invention to provide a drill string tool and a method for enhanced wellbore cleaning.